The Development of Leptoplastus Salteri (Callaway), and of other Trilobites (Olenidæ, Ptychoparidæ, Conocoryphidæ, Paradoxidæ, Phacopidæ, and Mesonacidæ)

Abstract

I. Introduction. The above-named species of Olenid was first recorded in 1874 by Callaway from the Shineton Shales of Shropshire, which he then claimed as of Tremadoc age. It was described from imperfect material as a Conocoryphe , which name in 1877 he corrected to Olenus , giving at the same time a description of the entire trilobite. A specimen for the first time exhibiting dorsal spines was in 1900 described by Dr. H. H. Thomas under a new name, Olenus mitchinsoni . An additional occurrence of the species was in 1905 recorded by Prof. W. G. Fearnsides under the name of ‘ Parabolinella salteri ’. It occurred in the upper part of his Nant-ddu or Bellerophon Beds, which follow next above the Dictyonema Band north-west of Arenig Fawr, and it is there accompanied by Asaphellus homphrayi Salter and Parabolinella triarthrus (Callaway), which species are also associated with it at Shineton. In 1907 I briefly described its development, and assigned it to a new sub-genus ‘ Leptoplastides ’. Finally, Mr. P. Lake redescribed it in 1919, under the name Leptoplastus salteri . It is now many years since, at Prof. Charles Lapworth's suggestion, I undertook the study of the trilobite fauna of the Shineton Shale. The first collection studied was that collected by Mr. Rhodes for H.M. geological Survey, and lent to Prof. Lapworth by the then Director-General. Among these were several slabs covered with minute examples of the fossil in question, in different stages of development; and the early stages seemed so strange and unexpected that a special study

Growth and variation in the Silurian proetide trilobiteAulacopleura koninckiand its implications for trilobite palaeobiology

“…In A. konincki, segment generation continued throughout ontogeny, and it is not possible to recognize 'completed' postcephalic exoskeletons. Hence neither Raw's (1925) nor Stubblefield's (1926) definition of holaspis applies to this species, although there is an ontogenetic transition analogous to the meraspid-holaspid transition. In other trilobites, meraspid pygidia are referred to as 'transitory' because they represent a dynamic balance between segment generation and anterior budding into the thorax (see Whittington 1959, p. 135).…”

“…This pattern ofposterior tagmosis provides a synapomorphy for all skeletonized (and possibly also soft-bodied) trilobites (Briggs & Fortey 1992;Fortey & Wittington 1989;Ramskold & Edgecombe 1991). In skeletonized trilobites, at least two major ontogenetic transitions are recognized, based on the development of the postcephalic region (Beecher 1895;Raw 1925). These are the appearance of articulation separating the cephalic and postcephalic shields (defining the protaspis to meraspis transition) and the termination ofaddition of segments to the thorax (defining the meraspis to holaspis transition).…”

Ontogeny and evolution of the Early Cambrian trilobite genus Nephrolenellus (Olenelloidea)

“…Phases of cephalic development.-Trilobite ontogeny is tradi tionally subdivided into the protaspid, meraspid, and holaspid pe riods (Beecher, 1895;Raw, 1925; reviewed by Chatterton and Speyer, 1997), based on the nature of trunk articulation. The si licified Nephrolenellus material described here is invariably dis articulated, and knowledge of early ontogeny is restricted to ce phalic morphology alone.…”

Ontogeny and shape change of the phacopid trilobite Calyptaulax

“…It has been speculated that earlier, morphologically distinct, less-mineralized larval forms may have existed, but whether the so-called phaselus larvae are indeed those of trilobites remains somewhat contentious and uncertain (Fortey and Morris, 1978; Hughes et al, 2006). The transition from the protaspid to the meraspid (Raw, 1925) period is marked by the separation of the cephalon from the trunk and the development of an articulation between the two tagmata (Whittington, 1957). Further molts produce a thorax of freely articulating segments (in contrast to the fused segments of the pygidium).…”